Microwave devices are used in many areas of technology. One application of microwaves is for example the heating of objects, such as microwave ovens in the household. A particular application concerns microwave devices as a fusing device in printing machines, whereby the image applied to a print substrate by a toner is fused to the print substrate by microwave irradiation and secured to it, whereby the toner is melted by the heating due to microwave energy and forms a substantial connection with the print substrate and is interlocked with it. The energy to be supplied for this fusing process is very high, caused in particular by the high speed with which the print substrate is passed through the microwave device, whereby, in the short fusing time that the print substrate spends in the microwave device, a high electrical power supply must be provided. Compared with the fusing of toner by means of rollers, or roller fusing, for the fusing by means of microwaves, or microwave fusing, approximately 20% to 50% more power per cm2 is required.